Misaka 19090 is one of the original production of Misaka Sisters originally used for the level 6 project, which was to be killed in combat by Accelerator for him to achieve level 6. But the experiments were canceled when Accelerator was defeated by Touma.
She was one of the few clones that stayed in Academy City after the experiment ended. It's revealed that Shinobu successfully implanted more emotions in her, making her able to display more emotions than the other Sisters (with exception to Last order, Misaka Worst and Dolly). She possibly has feelings for Touma like the other Sisters as she had tried dieting and reading up on shoujo manga.
In Toaru Kagaku Accelerator, the clone that visited him in the hospital and gave him Esther's message was implied to be her as she displayed more emotions than most other Sisters. She acknowledged and showed gratitude for Accelerator turning a new leaf and protecting the sisters, as well as showing concern for his well being, bearing no grudge towards him. When he claimed that he didn't do what he did to save them that time, she smiled as seeing through his cold facade, just like Last Order does. They had a rather friendly interaction.
